Let me ask you this – do you “trust” your partner to pick up something from the store that they said they would? Do you “trust” them to pick you up when promised? Do you “trust” them to not steal your money, and to not burn your house down? My point is this: trust is a matter of degree. How can you learn to trust again? The fact is, you already do.
Both partners have a responsibility in restoring trust in a relationship. The person who broke the trust, needs to genuinely do all they can to restore that trust. The other person needs to recognize that effort and ultimately forgive the other for past mistakes. You can’t hold a mistake over someone’s head forever, especially if they are doing all they can to make things better. That will only lead to bitterness for both of you.